1. Field of the Invention
The present invention relates to a scanning optical device for optically scanning and writing information, and can be properly applied to an image forming apparatus such as an electrophotographic copying machine having a photosensitive member or an electrophotographic printer having the photosensitive member.
2. Description of the Related Art
As a color image forming apparatus having image bearing members (as will be called the “photosensitive drums”) independent for the individual colors of yellow, magenta, cyan and black, there is known a tandem type color image forming apparatus for obtaining color images by exposing the individual photosensitive drums to laser beams to form electrostatic latent images, by developing the electrostatic latent images with toners of the individual colors, and by superposing the obtained toner images on a sheet-shaped recording medium.
In order to lower the cost and to reduce the size of the tandem type color image forming apparatus, on the other hand, there is known a scanning optical device in which, as exposure means for exposing the individual photosensitive drums to laser beams, a rotary polygonal mirror acting as deflective scanning means is commonly used for the plural light sources, and laser beams from a plurality of light sources are deflectively scanned simultaneously by one rotary polygonal mirror to carry out exposure by irradiating a plurality of photosensitive drums.
In order to improve the recording speed, on the other hand, there is known a multi-beam scanning optical device for recording information by scanning one photosensitive drum simultaneously with a plurality of laser beams. In this multi-beam scanning optical device, however, the photosensitive drum surface is simultaneously scanned at an interval in a sub-scanning direction with a plurality of laser beams. Unless the interval between the plural laser beams is a predetermined one in the sub-scanning direction (i.e., in the rotating direction of the photosensitive drum), therefore, the scanning lines have irregular pitches in the sub-scanning direction so that the image is defective. In the case of a resolution of 600 DPI (dots per inch), the laser beam has an interval as fine as about 42 microns. This makes it necessary to adjust the interval between the laser beams in the sub-scanning direction.
Here will be described the pitch interval adjustment of the multi-beams with reference to FIG. 11 and FIG. 12. FIG. 11 is a sectional view of the laser holder portion of the related art. FIG. 12 is a schematic diagram for explaining the pitch interval adjustment of the multi-beams of the related art.
In FIG. 11, a laser holder 101 has a holding portion 101a, in which a semiconductor laser (or a multi-beam laser) 102 having a plurality of light emitting points packaged therein is press-fitted and held. A printed-circuit board 103 is provided with a laser drive circuit and is electrically connected with the semiconductor laser 102.
To the leading end of the holding portion 101a, moreover, there is adhered a collimator lens 104 for converting a luminous flux into a generally parallel one.
When this laser holder 101 is to be mounted in the not-shown casing housing the optical parts of the scanning optical device therein, the pitch interval (i.e., the interval in the sub-scanning direction) P between two light emitting points 102a and 102b of the semiconductor laser 102 is adjusted by turning the holding portion 101a on its center of rotation.
More specifically, when turned in the direction of arrow B shown in FIG. 12, the pitch interval P of the two light emitting points 102a and 102b of the semiconductor laser 102 is reduced. When turned in the direction of arrow C shown in FIG. 12, on the contrary, the pitch interval P of the two light emitting points 102a and 102b of the semiconductor laser 102 is enlarged.
Here, the pitch interval P of the light emitting points is likewise adjusted even in case not a semiconductor laser having a plurality of light emitting points packaged therein but a plurality of semiconductor lasers each having one light emitting point are used to synthesize with using a prism for obtaining the multi-beam.
In JP-A-2000-280523, for example, there is disclosed an optical scanning device for performing an exposure by emitting multi-beams from a plurality of light source units for irradiating a plurality of photosensitive elements with optical beams and by deflectively scanning the plural photosensitive elements in a main scanning direction (normal to the rotating direction of a photosensitive drum) by one polygon mirror. This optical scanning device is configured such that the beam spot interval between two optical beams in a sub-scanning direction is adjusted by adjusting the amount of inclination on the center of an injection axis from the main scanning direction to the sub-scanning direction using an adjusting screw.
However, the related art thus far described has the following problems.
In the related art, the light source units of the individual colors are configured separately from each other. Therefore, the inter-beam distances of the multi-beams in the sub-scanning direction have to be individually adjusted to cause a problem that many adjusting steps are required.
The present invention has been conceived in view of the problems of the aforementioned related art and has an object to provide an image forming apparatus provided with a scanning optical device which can adjust the beam interval distance in the rotating direction of an image bearing member with less working steps, even if configured to have a plurality of laser illumination means each having a plurality of light sources.